So is Kong only a situational meat-eater?

I don't think so. In my mind, that pile of human skeletons was just from Kong being incredibly rough with them until they got killed. And in the commentary, Jackson specifically states that their Kong was a vegetarian, like real world gorillas.

As for Jack Prescott's assumption that apes only ate fruit, I'm guessing he based that thought off of gorillas and orangutans. Chimps on occasion eat meat, but I'm not sure if that was known at that time. I think chimpanzees still had the cute, cuddly image, where you wouldn't believe how much of a terror they could be haha.

Lastly, in Kong '33, he did indeed bite people. However, he never ate them. We always saw him pull their bodies out of his mouth and throw it on the ground. I think he just chomped down on people as a quick and efficient way to put them out of commission. Same with that guy Kong '05 disposed of.

All this, however is thrown out the window in King Kong Lives, where we do actually see him eat a person... Which I always thought as a mistake on the filmmakers parts. I don't think Kong should be a meat-eater. But that's just my opinion.